Section 300.20. Induced Currents in Metal Enclosures or Metal Raceways.
(A) Conductors grouped together.
Where conductors carrying Alternating current are installed in metal
enclosures or metal raceways, they shall be arranged so as to avoid
heating the surrounding metal by induction. To accomplish this, all
phase conductors and, where used, the grounded conductor and all
equipment grounding conductors shall be grouped together.
While this addresses how to eliminate inductive heating in cable runs,
it does not address eliminating them at or near the point of
termination. This has caused many to believe that inductive heating can
be generated on something as small as the surface of a bolt or water
pipe and can have dangerous consequences. It is developed in
ferromagnetic material by nearby current-carrying conductors. Two
electromagnetic properties combine to create this heat, hysteresis and
eddy currents.
Hysteresis is the tendency of
ferromagnetic material to hold remnant flux. If a ferromagnetic
material is subjected to a magnetic field it will retain some of that
magnetic flux. You can see this for yourself - put a magnet on a nail
for one minute. Remove the magnet and for some time the nail will act
as a magnet itself. If a ferromagnetic material is subjected to a
changing magnetic field (as with an AC circuit) the alternating field
causes losses within the material; this leads to a buildup of heat. AC
conductors produce an alternating magnetic field around the surface of
the conductors when they are carrying current. When all three phases
and the neutral of a three-phase system are bundled together, the
magnetic fields cancel each other. This is the reason that with
multiple conductors per phase, the conductors are not run in phase
groups, but instead run with all three phases and a neutral in a
conduit using as many conduits as there are conductors per phase.
Eddy currents are generated in conductive material when they are placed
perpendicular to the magnetic field produced by AC systems. They can be
generated in a flat piece of sheet metal or around the surface of a
water pipe. The inductance of ferrous material causes these relatively
low currents to generate considerable heat. Eddy currents are avoided
by not allowing a path for the current to travel (such as slotting an
enclosure surface).
These properties are most
often associated with transformers and reactors. Transformers avoid
eddy currents through the use of multiple insulated laminations that
eliminates the conductive path. Meanwhile hysteresis, due to the heat
it generates, remains the primary form of transformer losses.
The effects of this field can be cancelled on nearby objects by running
all phases, neutral and ground together. Bind the varied conductors
together and the fields cancel each other. Often times this can be
difficult at the point of termination, where phases, neutral and ground
must be separated. It can also be difficult at the point where the
conductors enter or exit an enclosure. At any point the conductors are
separated, no matter how small, inductive heating can develop.
How do you avoid inductive heating? At any point the conductors
separate, be careful of the components you select for bracing. Avoid
using ferromagnetic materials in favor of aluminum or fiberboard.